Seismic techniques are frequently used during hydrocarbon-exploration expeditions to identify the existence, location and extent of reservoirs in subterranean rock strata. However, whilst seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them. This is especially so for pore fluids which have similar mechanical properties, such as oil and seawater. It is therefore generally necessary to employ other survey techniques to determine whether a previously identified reservoir contains oil, or just aqueous pore fluids. One technique is exploratory well drilling in the region of potential interest. However, this is expensive and time consuming. An alternative technique recently developed is the controlled source electromagnetic surveying (CSEM) technique, such as described in GB 2 382 875 [2].
CSEM seeks to distinguish oil- and water-filled reservoirs on the basis of their differing electrical properties. This is done by transmitting an EM signal into seafloor, generally using a horizontal electric dipole (HED) source (transmitter), and measuring the response at EM receivers (detectors) for a range of distances from the source. Since hydrocarbons are more resistive than seawater, the presence of a hydrocarbon-bearing reservoir will, in general, lead to stronger EM fields than would be the case if the reservoir contained only seawater. This is because the highly conducting seawater attenuates the component of the EM signal passing through the reservoir more than would be the case if the reservoir contained hydrocarbon.
One proposed technique for CSEM surveying is to tow an EM source over a line of receivers and to directly compare response amplitudes for source-receiver offsets which are over a target with corresponding response amplitudes for source-receiver offsets which are not from over the target so that any enhancement in amplitude can be observed [1].
In practice, however, the interpretation of CSEM survey results is not so simple. In particular, it has been recognised that surveys must be carefully planned to reduce the likelihood of certain types of large-scale background structure giving rise to false-positive indications of hydrocarbon reservoirs. One way of doing this is to obtain survey data for a range of relative orientations between source and receiver, e.g., by towing a HED source over a deployed array of receivers. Data from receivers which are arranged inline with the HED source (i.e., on a line parallel to and passing through the axis of the HED axis) are more sensitive to the presence of thin resistive layers indicative of hydrocarbon-bearing reservoirs. Data from receivers which are arranged broadside to the HED source (i.e., on a line perpendicular to and passing through the HED axis), on the other hand, are more sensitive to characteristics of the large scale background. A comparison of data from both orientations can be used to reduce false-positive indications [2].
Analysing both inline and broadside data from a target region of interest has proven to be a reliable way of reducing the likelihood of ambiguous interpretation of CSEM data. However, there are still some classes of subterranean strata configuration which are not hydrocarbon bearing, but nonetheless produce characteristic CSEM responses which are similar to those associated with hydrocarbon reservoirs. For example, subterranean rock formations having significant degrees of vertically anisotropic conductivity (differing conductivity in the vertical and horizontal directions) can provide inline and broadside responses which are similar to those seen with hydrocarbon reservoirs. This means a vertically anisotropic rock formation can give rise to a false identification of hydrocarbon. Furthermore, in the event a vertically anisotropic rock formation does bear a hydrocarbon reservoir, the nature of the reservoir will be incorrectly determined. In particular, background anisotropy will lead to an underestimate of reservoir depth, and an over estimate of reservoir resistivity [3]. Thus a deep reservoir containing a relatively low fraction of hydrocarbon, and so of reduced economic interest, can be mistaken for what appears to be a commercially more interesting shallower reservoir with a higher hydrocarbon carbon (i.e., higher resistivity).
There are a number of subterranean strata configurations which can display significant degrees of vertical conductivity anisotropy. It is often displayed in sand and shale layers, and grain alignment of clay minerals in sedimentary layers can also lead to anisotropic conductivity, as can alignment of fractures in subterranean layers. Thus subterranean strata configurations such as these can lead to false positive indications of the presence of hydrocarbon, or an over estimate of the amount, and ease of recovery, of any hydrocarbon that is present. There is therefore a need for techniques which further reduce the likelihood of ambiguous interpretation of CSEM data to help avoid expensive drilling and further survey programs of regions which have been wrongly indicated as containing commercially useful quantities of hydrocarbon.